Considering that Roland is returned to the point just before he met Jake (while searching for the Tower) I think he is doomed till he can fix his first and most horrid action in the story. That being the allowing to die of a complete innocent in his pursuit for the Tower.
Everyone else that died had some sort of shadow on their soul. Jake, on the other hand was clean at that point of his story. I feel that as soon as he sacrifices the boy the rest of his destiny is written.
But, I may be completely wrong